Demodulator device



April 17, 1956 F. v. HUNT ET AL 2,742,566

DEMODULATOR DEVICE FREDERICK V. HUNT NORMAN B. SAUNDERS ROBERT E. KIRKLAND :ATTORNEYS April 11, 195e Filed Oct. 9, 1951 F. v. HUNT ET AL 2,742,566

DEMODULATOR DEVICE 3 Sheets-Sheet 2 FIG.2.

UTILIZATION DEVICE INVENTORS FREDERICK V. HUNT NORMAN B. .SAUNDERS ROBERT E. KIRKLAND ATTORNEYS April 17 1956 F. v. HUNT ET AL 2,742,566

DBMODULATOR DEVICE Filed OCI.. 9, 1951 3 Sheets-Sheet 3 FIG. 3.

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LlMlTgR lOl UTIL IZATION DEVICE INVENTORS FREDERICK V. HUNT NORMAN B. SAUNDERS ROBERT E. KIRKLAND ATTORNEYS United States Patent signors to the United fStatesfof America as represented by the Secretary of the Navy Application october 9, 19s1,`ser`iai No. 250,534 zrclaims.- (Crass-ao) 'This invention broadlyrelates to' signal responsivedevices in general, particularly relates to signal demodulator devices'loperative to improve the signal lto'noise ratio in received signals which may be accompanied' by considerable undesired noise components, and specilcally relates to improved sonar systems.

In practically all signal reception systems intended to receive low level signals and amplify them for subsequent utilization, the minimum'usable input signal level is ultimately limited by the amount of noise received with the signal itselfor generated within the first stages of the reception system. Much of such noise isiof random frequency distribution. However, under certain` circumstanceslthe noise present may be more or less concentrated atcertain frequencies or in certain bands of fre'- quencies; i'

In some instances, signals which'the'mselvesare of a narrow band nature and whichmay `be handled by narrow band circuits-may occur anywhere'overa-relatively wide frequencyrange in which` case-the receiver must be capable of -receiving signals-"over this entirerange. In many. such cases, although the signal mayj'override the noisein its'y immediate vicinity, the' total noise occur-y ringrin the entire wide range mayengulf the signal.' For such situation, conventional detectors `as employedin receiver amplification systems would respondto the entire range and yielddetectedsignals which are barely discernible, if'atall, through the noise.

It is Iaccordingly an object ofthe present invention to provide an' improved demodulator for wide band operationY with relatively low signal to noisel ratios.

It is anotherobject of the present nventionto' provide apparatus'for improving the signal to noisev ratio in' signal reception systems;

Another object of the present invention is' to provide a' method' of signall demodulation particularly suited' for operation under conditionsof 'low signal to noise ratios.

Anotherv object of the present invention is to provide. apparatus for improving the signal to noise ratio in signal reception systems operative under conditions of low signal to noise ratio.

The invention iinds particular'application-for sonar use. In sonar applications for detecting the'presence of an lunderwater objector target a sound signal or pulse of; al sharply tuned frequency is sent outtowardthe target andthe echo of this signahas rellected from the target, is receivedf By suitable orientation the location ofthetarget'can be' determined. Frequently, the echosignal may be immersed in background noises of maskingy intensity making it difficult to'identify the echo.

It is known that the discriminating senseof the ear for pulse signals arises'from its ability to concentrate its response in a narrow band of frequencies which forms only a small part of the larger range of frequencies in the rsound heard by the ear. The earselects a bandwhich corresponds to'that in which the desired signal is most concentrated and audibleover'the rest'of the sound, re-

2,742,566 Patented Apr.. 17, 195e gardless of the intensityk ofthe sound outside that "band, By this process the ear is able to recognize a signal impulse that is immersed in a background noise of masking intensity, so long as the signal dominates 'in a small band ofthe background noise. An object of this invention is to'pr'ovide sonic receiving equipment that operates' in a rice ' manner similar-to the way inwhich the human ear'operates for recognizingisonie types of signals in the presence of noisewhich exceeds the; signals in average intensity.

Another object of the invention is V'to provide sonar receiving equipmentcapablevof rresponding to a short signal of 'a' critical 'frequency within a muchwider band.

It is a broad object'of this invention to provide sonic receiving equipment capable of discriminating between a sonic signal impuls'esand background noises whenthe signal to noise intensity-ratio is'high and also when the background noises have a generally greater intensity than thesignal. Obviously such equipment will have a range and effectiveness that'are much lgreater than that ofv conventional lsonic receiving equipment which always requires a comparatively high signal to noise .intensityratio through the' full'receivable band for satisfactory operation. f

The invention has especial advantages in connection with receiving signals fsubjeetedto marked Doppler shifting.v Thus,` forexatnple, in`V the detection of an underwaterftargen'the narrow 'frequency-bandwidth of the transmitted signal and of f the received echo-signal will be the same'only if the target is knot moving relative to the oper.- atingf'pointfof transmissionand reception. Such conditions' rarely prevailg, instead,V the frequency of the echo will befshiftedfby yan amount that is a functionof the relative movement of the target and the aforesaid operatingpoint. Since-theV amount of this relative movement Ais'gtriei-ally unknown, ai' receiver for -theecho must be Whose frequencyhas beenl shifted by an unknown amount, although the echo'itself `possesses a relatively narrow frequency bandwidth. Under such circumstances. the receiver receives noises Vthroughthe broadV range' of 'the' receiver whichmightfordinarily mask out .the echo; but in accordance with theA invention, .the signal to noise ratio isimprovedlsoas to render thesignal Vdiscernible or useful. t

A further and7 important object of the invention is to provi'clesonic equipment capable 'of converting a signal impulsethat'is too short to be recognized by the human ear into one-that is readily recognized. v

Stillanother objectfof v the invention' is to provide sonic receiving equipment capable `of recognizing a signalimpulse thatistoo short'to -b`e perceived by the human ear and is:also immersed'in enveloping background noises of greater average intensity.l i With` these and other objects in View, as will hereinafter-'more 'fully appear, and which will be more particularlyr pointed 1out'inthe appended claims, reference is now` made to the following description' taken inv connection with'itheac'companying drawings in which:

Fig;l 1 isna schematic diagram'of a basic embodiment of the featuresof the present invention.

Fig..2'isaivariant'embodiment of a component circuitwhichlcan be employed in the apparatus of the present inventions Fig: 3A is-a. modification of= the'basic embodiment of Fig. l.A

In accordance with the basic` concept of thepresent invention an improved demodulator system is provided which possesses numerous. advantages particularly under operating conditions'where thesignal to noiseratio is'low, for example, less than unity. Suc-h a demodulator'system nds great utility in sonar applications, for example, where 3 the desired signal, even though it possesses a relatively narrow bandwidth, may be shifted by Doppler effect to fall anywhere over a rather wide frequency range. In the present demodulator system, the wide frequency range is divided into a plurality of relatively narrow bands or chan nels, each of which has a bandwidth which is no more than adequate toY contain all or a major part of the spectrum of the desiredsignals. The output of each channel'is demodulated independently and then combined with the outputs of the other channels into a single output circuit. An important feature of this device is that the desired signal thus obtained from any one of the channels is employed to control the sensitivity of demodulation in all channels', reducing the sensitivity in channels where the desired signal is absent or the signal to noise ratio is exceptionally low, thereby to further reduce the overall noise present in the combined output.

With particular reference to the drawings herein in which likereference characters designate corresponding parts throughout the several views, there is shown in the basic embodiment of Fig. 1 an improved demodulator system constructed in accordance with the teachings of the present invention. This system includes a suitable input coupling circuit, such as the transformer 11, having coupled primary and secondary windings 12 and 13 respectively. Primary 12 is connected to signal input terminals 14 whereas secondary 13 is connectedto the grid 15 of an electron tube 16 through the rcoupling circuit comprising in the main the capacitance 17 and resistance 18. Resistance 18 is further placed in series with resistances 19 and 20, the latter being connected to a suitable source of (B+), 21,A whereas the secondary winding 13 is connected to the juncture point of resistances` 18 and 19 through the resistance 22. Also connected in the .grid circuit of tube 16 is a suitable noise-limiting means such as the opposed biased unilateral impedance devices 23 and 24. The inclusion ofsuch limiting devices is desirable to limit noise pulses and otherwise maintain a'greater uniformity in voltage level in the output of tube 16. The unilateral impedance device 23is placed in the circuit with its anode 25 connected to the grid 15 and its cathode 26 connected to the juncture point of resistances 19 and 20. Unilateral impedance device 24 is placed in the circuit with its cathode 27 connected to the grid 15 and its anode 28 connected to ground.

Tube 16 which is typified as of the beam power variety such as a type 6V6 has its cathode connected to ground through a suitable cathode biasing resistor 29, its screen connected to a suitable positive supply, and its anode connected to a positive supply through an impedance element 30. Normally this impedance element could comprise a choke having aninductance of suitable magnitude for the frequencies' involved.

Coupled to the anode of tube 16 is a plurality of frequency selective circuits represented in Fig. l by the numerals 31, 32, 33, and 34. It is understood that although only four such circuits are shown in Fig. 1 to avoid undue circuit complexity, actually many more circuits would normally be used. These circuits are all of the parallel resonant variety, individually tuned to selected different frequencies throughout the full range over which signals applied to terminals 14 may occur. Each parallel resonant circuit has a somewhat restricted bandwidth tuned to a fundamental frequency slightly different from that of the circuits tuned closest to it, but the range of all of the circuits together corresponds to that expected to be applied to the terminals 14. Preferably, each circuit has a reasonably high Q, and preferably the inductances are of the same value.

The circuits 31, 32, 33 and 34 are coupled to the anode of tube 16 through the capacitance 35 and suitable isolating resistance elements 36, 37, 38 and 39 which are included to minimize interaction between the resonant circuits,

Each of the parallel resonant circuits is connected to a demodulator here shown as 'of the infinite impedance type with triode tubes 40, 41, 42 and 43. Signal input is to the grid of each of these tubes, connection being made directly across' the appropriate resonant circuits 31, 32, 33 and 34. All anodes 44, 45, 46 and 47 of the tubes are connected together and through resistance 48 to the source 21 of (B+) potential. Similarly, cathodes 49, 49a, 50 and 51 are connected together to a common cathode biasing network. This network as shown in Figi comprises the variable resistor 52 shunted by the bypass capacitance 53fand a bleeder resistance 54 connected to (B+) 21. Signals developed across the demodulator load resistance` 48 are applied through coupling capacitance 55 and a dual section low-pass filter, including resistances 56 and 57 and capacitances 58 and 59, to the grid of an amplifier tube 60; the connection including a potentiometer 61 that provides an effective means for selecting the amount of filtered output signalvapplied to the amplifier tube. Signals realized at the anode ,of tube 60 are applied via a suitable output coupling circuit 61 to a utilization device of more or less conventional form.

In the operation ofy this overall circuit` of Fig. l, tube 16, in the absence of applied signals to terminals 14, must be considered as being operative with substantially zero bias by virtue of the conductive path from (B+) through resistances 20, 19 and 18 to grid 15. Diode action between the grid and cathode of tube 16 thus effectively clamps the grid 15 at a potential slightly higher than cathode potential. Substantially consistent bias potential thus is maintained holding a fixed reference level at the anode of tube 16. When input signals are applied to terminals 14, they appear across secondary 13 and are clipped in both. plus and minus polarities between the opposed impedance devices 23 and 24. The signals are then coupled through the tube 16 and capacitor 35 to the frequency selective, circuits 31, 32, 33, 34, etc. Y Thus in accordance with the frequencies of the applied signal, appropriate one or ones of these frequency selective circuits will be excited into damped oscillation. The `frequency selective circuits 31, 32, 33, 34, etc. are stepped or graded in increments through the range of frequencies that are to be received. If it is assumed that a pure tone signal of K cycles per second is being rcceived, and that this frequency occupies butl a narrow part of the band covered by the cricuits considered together, then the signal will strongly excite the particular circuits 31, 32, 33 or 34, etc. tuned closest to it, the other circuits being weakly excited, if at all. In a sense, the different tuning of the circuits provides a discriminating or selective effect channeling the signal through a selected tuned or filter circuit. The graded frequency tuning of the circuits 31, 32, 33, 34, etc. to a series of adjacent frequencies in a chosen range provides signal selection or noise rejection on a frequency basis. Also by providing the relatively high Q as above specified, an input signal which may in some instances be of an intermittent nature, is effectively lengthened which in some cases is desirable.

l The infinite impedance demodulators or detectors, for example,.detectors 40, 41, 42 and 43 are biased near cutoff by adjustment of theresistance 52 so that when broad-band noise of the type normally to be expected in use of the apparatus is on input terminals 14, practically no output is obtained from tube 60. 4In this connection itis to be realized that such cutoff biasing is possible because of the bleeder resistance 34 connected to (B+-)which can thereby maintain the cathodcs of tubes 40, 41, 42 and 43 at a positive potential if desired even with these tubes cut off.

as 31,732, 33, and 34, according to frequency, will be excited into'relatively large oscillations. When this ocn af'wtagsee curs, the appropriate 'innite 'impedance demodulatory will conduct aheavier plate currentthan before to produce a *voltage drop 'acrossA anode 'circuit resistance :48lowerin`g the potential at the anodes of the tubes. Simultaneously the current through resistance 52 increases.v

The increaseddropacrossthe resistancef4`8 manifests itself' at the output circuit 61 and a utilizable' signal is producedresponsive to thatfreceived. Moreover, the i11-v creased drop across resistance 52 provides additional bias on the tubes 40, 41, -42'and43`to drive them further beyond cut-off and.therebyreduceftlrenoise contribution' of each demodulator `in the combinedoutput applied` to the grid of tube 60.

It is thus seen that this basic demodulator` circuit provides notonly noiselimiting on an' amplitude basis but also noise-to-signal'ratio-reduction on a'frequency basis wherein signals presentati one frequency effectively reduce the'overall system response to noise signals at other frequencies. n

`While the' invention is of broader application, its teachingsV are particularly applicable" toisonar use'. For such use asmany astwenty ormoreV resonant or tuned circuits, such as 31; 32, 33 `and 34 may be provided' tuned to a series of adjacent frequencies inthe audio-spectrum between 600 and 1000 cycles `per' second, being distributed approximately uniformly on a logarithmic scale, each tuned circuit havingan inductance coil of .69 henry. The resistor, such as 36', 37, 38 and-39, associated-with eachj tuned circuit is of a suitable valueto so isolate the asso# ciated tuned circuit that a voltage is developed across'the circuit atresonancethat is approximately two-thirds of that required tor drive the tube 16 toffull output; 'The sharpness of resonanceof-the individual tuned circuits produces a considerable improvement in discrimination,-V

especially since theI desiredsharp sgnal'comp'etes in a single tuned circuitwith only abouti/20 of the total *noise energyr accepted-by the complete set of'twenty tunedecircuits,y on the assumptionf'offapproximatelyV uniformfdistribution of the noise. yEvenfwith an unevendistribution of noise, the discrimination will be considerably enhanced.

With reference now to Fig. 2, a modification in the basic circuitis shown which under' certain circumstances may* oifer further improvement in noise rejection.- The ape paratus of `this circuit by comparison to Fig. 1 shows only-V two resonant circuits and demodulatorslto avoid'unduey complexity, it being Yunderstood that in Aa complete apparatus there would be required: as manyl more such channels; ln this circuit of Fig. 2 input signals are ap'-` plied to input terminals 70. It" isf desirable that am'- plitude limiting be employed as with Fig. 1, to which end:

a limiter '70#1 is included. Terminals 70 are connected through limiter 70-a to the primary winding 71'oftransformer 72'Which transformeralso hasa secondarywinding 73 providing excitation-for series resonant frequency selective circuits comprising, for example, capacitance 74'- and inductance`75. Actually the series resonant circuit of capacitance 74r and inductan'ce 75 is' equivalent inff'requency selection to one of the resonant circuits'such `as- 1 tance '75 is'connected by wayof couplingcapacitance y70 to the grid 79 of electron tube 80 which as shownis of f the triode variety having, in additionV tothe grid 79, anode and'cathode electrodes. A second triode type electro-n tube 81 has its grid 82 connected by way of the coupling circuit of capacitance 83' and resistance S4 and potentiometer 85' to the juncture point of capacitance 74'and inductance 75. v

Irfthe'grid-circuit of tube 80 is disposed aigridfreturn resistancers' and apotentiomete'r 87. PotentiometerV 87 is connected'between rasuitable source of (B+-)88 and ground and by virtue of thevariable tap thereof may be employed to 4rcontrol'the D.`C. potential existent-at the grid 79.

The anode 89 of tube 80`is connected to (B+) 88"by way of resistance-90. The cathode of tubeis connected to-th`e anode of tube' 81, this Common connection. beingby-passedfto ground vbyscapacitanee 91 andc0nduc tively connected to-(B-[') 88 through resistance 92; In; turn the cathode of tithe` 81 is grounded'. The anode 89 of'tube 80is connected', through utiliztionrcircuit 93, to sortie suitable-form of utilization device not shown: In normalfcircuit operation, electron tub'eliwhich is possessed'normally'of near'zero bias is'conductive to some extent, formingxwith resistance`92 a voltage divider across (B+) to maintain' 'the "cathode of tube 80' at Vsome po'tential'whicl-fislower than (B+) S8'. With'such a condition lpotentiorneter'r 87" is" adjusted" to hold tube 80' at cut-'ofi ift-the absence of an appiied'signal at terminals '70; Signalsthien developedacross the inductance 751as a result'of input' signals supplied to input terminals 70 raisethe grid79`of'tube 80 above cut-off potential to bring tube 30 to conduction and provide output signals which are developed' across resistance 90. This action is modied through the operation of tube Sl'which also receives at its grid 02 a fraction' of the signal developed across inductance 75A; as selected by a variable tap on potentiometer 85. These signals when;

appliedth'rough the coupling circuit capacitance 33y and resistance 04 tothe normally zero biased tube y31 result in grid circuitv conduction in tube Slto develop a grid leak biasing voltage across capacitance 83 by familiar process. This grid leak bias voltage causes the grid 82 to assumean average'potential which is negative'witlr respect to the potential at the cathode of tube 01, 'thereby eec'tiv'ely increasing the average anodev resistance of tube 3i and raising the potential at the cathode of tube 30.` This action increases the negative bias on tube 801 so that it is no longer operating at cut-olf and .hence less responsive to small amplitude noise. n l

In the complete circuit in accordance with 2, aplurality of dual triodes such as the electron tubes S0 and 8.1 would be employed, all having independent biasing elements, all employing the same single anode resistancev 90, and all receivingthe signals from secondary 73 through individual series resonant circuits. As further exemplication, the anode of tube Sil-a is shown con as before a suitable form of input coupling means such asV the transformer 101 to receive input signals-across its primary 102 as applied to the input terminals 103. Limiting may be provided if desired by tlie'limiter 103-rz inserted be.

tween the input terminal 103 and the transformer i041. The secondary 104 of transformer 101 is connected to a plurality' of band-pass lters v105, 106, and 107 which, for example, may be of thel series'of parallel resonant type as shown inthe preceding'gures or the like. v The primaryy requirements are that they be set individually to the desired typical frequency so as to be graded through the range of frequencies to be received, and have a suitable Q factor. For sonar the range may be from 500 to 1000 cycles and the Q about 30.-

The output of each band-pass filter is connected to a demodulation device, the type as herein shown including a pair of unilateral impedance elements typied by components 108 and 109 for the band-pass lilter 105. By way of example, impedance element 108 is an electron-tubey of diode ktypehavingjan anode electrode 110 and a cathode electrode 1,11. The anode of tube 110 is connecteddirect to the output of `theband-pass filter S whereas the cathode 111 is connected to ground through resistance 112 in series with resistance 113, the latter being by-passed by capacitance 114. The anode 115 of impedance element 109 is likewise connected to the output of band-pass filter 105. However, this connection is made through the primary winding 116 of a coupling transformer and also through the potentiometer 117. The cathode of impedance element 109 is connected `to ground through resistance 119 in series with the previously mentioned resistance 113 and capacitance 114.

The primary winding 116 is part of a coupling transformer 119 which additionally includes a secondary winding 120 and an electrostatic shield 121. The secondary winding 120 is connected through isolating resistance 122 to some suitable form of utilization device which is not shown in Fig. 3. Similar connections of the second set of unilateral impedance elements 123 and 124 to the output of band-pass lter 106 and the third set of unilateral impedance elements 125 and 126 to the output of band-pass filter 107 are made, the outputs from all three sets of unilateral impedance elements being combined into a single channel appearing across load resistance 127.

In the operation of this circuit of Fig. 3 when a lter output signal is realized for example from band-pass filter 105, a rectified voltage is developed by unilateral impedance element 108, providing a similar voltage across resistance 113 which also appears at the cathode 118, being applied through the resistance 119. This olset biasing voltage developed in response to signals from band-pass filter 105 produces the same sort of variation in threshold level as was obtained in the devices shown in Figs. 1 and 2. That is, responsive to received signals of' desired frequencies the detector threshold level is altered to reduce the sensitivity of the system to noise. The time constant determined by capacitance 114 and resistance 113 is such that the bias variation will follow relatively slow changes but wil not respond so rapidly as to eliminate the rectification of short pulse signals by the unilateral impedance element 109, Conduction by unilateral impedance element 109 provides a signal across primary winding l116 which is coupled to the secondary winding 120 to appear in the output for delivery to the utilization device.

From an examination of the circuits presented above it can be seen that signal demodulation performed thereby provides for certain types of signals a measure of noise reduction not available with detection systems previously known.

It should be understood, of course, that the foregoing disclosure relates to only typical embodiments of the invention and that modification or alteration may be made therein without departing from the spirit and the scope oi the invention as set forth in the appended claims.

What is claimed is:

l. Demodulator apparatus adapted for detecting a sharply tuned signal with apparatus having wide bandwidth operation comprising, a signal input circuit, limiting means for amplitude limiting input signals applied to said signal input circuit, a plurality of narrow bandwidth series resonant tuned circuits, each being of relatively high Q and responsive to a narrow bandwith portion of the wide band desired, a plurality of demodulators for individually demodulating the signals from the resonant circuits and each comprising a multi-electrode electronic tube having a cathode electrode and a grid connected to the output of one of said tuned circuits for receiving the signals therefrom, each of said demodulators further including a multi-electrode electronic tube in series relationship with the cathode electrode of each of said demodulators responsive to undemodulated input signals from the corresponding one of said plurality of tuned circuits and operative to provide negative bias at cut-off for said tirst-men- 'Niv tioned electronic tubes in absence of desired signals and in excess of cut-off in presence of desired signals, and means for.combining the demodulated signals for all of said demodulators.

2. Demodulator apparatusadapted for detecting a narrow band signal falling somewhere between the limits of a much wider band and in the presence of objectionable noise distributed between the limits of the much wider band, said apparatus comprising: a signal input circuit; limiting means connected in circuit with the input side of said signal input circuit for amplitude limiting input energy applied to said signal input circuit; a plurality of narrow bandwidth series resonant tuned circuits each having a relatively high Q and responsive to successive contiguous narrow bands of frequency between the limits of the aforesaid much wider band, each of said plurality of tuned circuits connected across the output side of said signal input circuit and each having a pair of output terminals; a corresponding plurality of demodulators each connected to the output terminals of a corresponding one of said tuned circuits for individually demodulating the signals from the corresponding tuned circuits; each of said demodulators including a first and a second multi-electrode electronic tube each having at least a plate, a control grid, and a cathode, the cathode of said tirst tube connected to the plate of said second tube; a corresponding plurality of coupling condensers connecting the control grid of each of said first tubes to one of the output terminals of the corresponding one of said tuned circuits, the cathode of each second tube being connected directly to the other terminal of the corresponding tuned circuit; a corresponding plurality of variable voltage dividers separately connected across the output terminals of each of the corresponding tuned circuits; a corresponding plurality of grid-leak biasing means connecting the control grid of each second tube to the output of the corresponding voltage divider; a corresponding plurality of bypass condensers separately connected in shunt across each second tube; a common power snpply; a common load connected between said power supply and the plate of each of said first tubes of each of said demodulators for combining demodulated signals of all of said demodulators;.each of said demodulators further including a separate adjustable voltage divider connected across said power supply and to the control grid of its first tube for applying a positive potential thereto and a separate load resistor connected between said power supply and the plate of said second tube; whereby in the absence of signal output from the corresponding one of said tuned circuits the resultant bias on the first tube of a demodulator resulting from conduction through the second tube and due to the positive potential on its control grid obtained from the corresponding adjustable voltage ldivider connected across said power supply is just suiiicient to bias the iirst tube at cutoff and when a signal output is derived from the corresponding tuned circuit the bias on the second tube is driven negatively for making the demodulator less responsive to small amplitude noise accompanying the signal output from the corresponding tuned circuit.

References Cited in the tile of this patent l UNITED STATES PATENTS 

